Required Practical: Investigatng Water Potential (AQA AS Biology): Revision Note

Required practical: investigating water potential using calibration curves

• It is possible to investigate the effects of immersing plant tissue in solutions of different water potentials and then use the results to estimate the water potential of the plant tissue itself

• The most common osmosis practical of this kind involves cutting cylinders of potato and placing them into solutions with a range of different water potentials (usually sucrose solutions of increasing concentration – at least 5 different concentrations are usually required)

Method

1. Cut the required number of potato cylinders (one for each of the solutions you are testing – or more than one per solution if you require repeats)

2. Cut to the same length, blot dry to remove any excess moisture, then measure and record their initial mass

3. Placing the potato cylinders into the solutions

4. Leave in the solutions for a set amount of time (eg. 30 minutes), usually in a water bath (set at around 30^o)

5. Remove the cylinders and dry to remove excess liquid

6. Measure and record the final length and mass of each potato cylinder

Analysis

• The percentage change in mass for each potato cylinder is calculated

Interpreting the results

• A positive % change in mass means the potato gained water by osmosis, so the solution had a higher water potential

  • The cells became turgid, and the potato felt firm

• A negative % change means the potato lost water, so the solution had a lower water potential

  • The cells became flaccid, and the potato felt floppy

• The largest mass decrease occurs in the most concentrated sucrose solution, where the water potential gradient is greatest

  • Cells may be plasmolysed

• If there is no change in mass, the solution was isotonic with the potato meaning that water moved in and out equally and there was no net movement

Graph intercepts

• The concentration of sucrose inside the potato cylinders can be found if a graph is drawn showing how the percentage change in mass changes with the concentration of sucrose solution – the point at which the line of best fit crosses the x-axis is the concentration of sucrose inside the potato cylinders

• A positive percentage change in mass indicates that the potato has gained water by osmosis (net movement of water from the solution into the potato)

  • meaning the solution had a higher water potential than the potato

• A negative percentage change suggests the opposite

Plotting data

• Plotting data from investigations in the appropriate format allows you to more clearly see the relationship between two variables

• This makes the results of experiments much easier to interpret

• First, you need to consider what type of data you have:

  • Qualitative data (non-numerical data e.g. blood group)

  • Discrete data (numerical data that can only take certain values in a range e.g. shoe size)

  • Continuous data (numerical data that can take any value in a range e.g. height or weight)

• For qualitative and discrete data, bar charts or pie charts are most suitable

• For continuous data, line graphs or scatter graphs are most suitable

  • Scatter graphs are especially useful for showing how two variables are correlated (related to one another)

Tips for plotting data

• Whatever type of graph you use, remember the following:

  • The data should be plotted with the independent variable on the x-axis and the dependent variable on the y-axis

  • Plot data points accurately

  • Use appropriate linear scales on axes

  • Choose scales that enable all data points to be plotted within the graph area

  • Label axes, with units included

  • Make graphs that fill the space the exam paper gives you

  • Draw a line of best fit. This may be straight or curved depending on the trend shown by the data. If the line of best fit is a curve make sure it is drawn smoothly. A line of best-fit should have a balance of data points above and below the line

  • In some cases, the line or curve of best fit should be drawn through the origin (but only if the data and trend allow it)